1. Field of the Invention
The invention relates to fluid-operated brake actuators for vehicles and more particularly to service and spring brake actuators combined in tandem and having a spring brake actuator rod.
2. State of the Prior Art
An air brake system for a vehicle such as a bus, truck or the like typically includes a brake shoe and drum assembly which is actuated by means of an actuator assembly operated by the selective application of a fluid such as compressed air. Conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and an emergency or spring brake actuator which causes actuation of the brakes when air pressure has been released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This is often referred to as the spring brake. Typically, the spring brake actuator is disposed in tandem with the service brake actuator.
The spring brake actuator is typically divided into two chambers separated by a rubber diaphragm and pressure plate, with the spring in one of the chambers acting between and end wall of the spring brake housing and the pressure plate. When full pressure is applied to the opposite chamber, air pressure acting against the diaphragm and pressure plate compresses the spring. In many applications, a spring brake actuator rod is held in a retracted position by a relatively small return spring. In newer applications, the spring brake actuator rod is integral with the pressure plate and held in a retracted position by the air pressure.
In both designs, the spring brake actuator rod thus does not affect the normal operation of the brake. The service chamber is typically divided into two chambers by a diaphragm. Depressing the brake pedal during normal driving operation introduces compressed air into one of the chambers of the service brake actuator which, acting against the diaphragm, causes a service brake push rod in the opposite chamber to be extended and the brakes to be applied with an application force proportional to the air pressure in the service brake actuator.
In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator, the brake will be mechanically activated by the force of the strong compression spring acting on the spring brake actuator rod which, in turn, acts upon the service brake push rod to apply the brakes. Thus, the spring brake portion serves both as a parking brake and an emergency brake.
In tandem actuator assemblies, the spring brake push rod typically extends from a chamber in the spring brake portion, through an aperture in a wall separating the spring brake actuator from the service brake actuator, and into a chamber in the service brake portion. Because at least one of the adjoining chambers is usually pressurized, a seal is provided at the aperture around the push rod comprising one or more O-rings positioned in annular channels in the wall around the aperture.
When pressure is released from the spring brake actuator, the spring and diaphragm extend significantly, expanding the volume of the spring brake actuator chamber containing the spring. Means must be provided for allowing air to enter the expanded volume of the chamber. Conversely, when the spring is retracted, and the volume of the chamber contracts, means must be provided for evacuating air from the chamber. In many prior brake actuators, the chamber containing the spring is simply open to atmosphere through ports in the chamber housing. However, this allows dirt, salt, moisture and other unwanted material to enter that chamber through the ports. With the advent of hollow actuator rods containing caging tools, the presence of foreign material within the actuator rod has become an increasing concern of brake designers.
Some prior designs have attempted to address this problem by providing a breather tube between the push rod chamber of the service brake actuator and the spring chamber of the spring brake actuator, as in the U.S. Pat. No. 4,960,036 to Gummer et al. In this design, the push rod chamber of the service brake actuator must be vented to atmosphere for normal application of the brakes. Venting of the push rod chamber occurs either through a separate vent, or through the external opening carrying the push rod. Thus, the spring chamber of the spring brake actuator remains open to atmosphere, and is still exposed to moisture, dirt and grime, albeit through a longer passage than a fully vented spring chamber. This longer passage may also prevent the spring chamber from drying as effectively as a sealed or more fully vented spring chamber.